Processes employing beds of fluidized solids in modes of fluidized suspension or fluidized transport are well known. A particularly well known example of such a process is the fluidized catalytic cracking (FCC) process for the conversion of gas oils and heavier boiling hydrocarbons into lighter hydrocarbons. In most applications where a large diameter vessel or conduit confines the fluidized particles, it is essential that a good distribution of the gaseous fluidizing medium be obtained over the entire cross-section of the vessel or conduit. Good distribution of gas is necessary to evenly convey the particles when the fluidized bed is in a transport mode. Moreover, the introduction of a gas reactant, typically air, into the bed of fluidized particles increases the demand for even air distribution. A poor distribution of air promotes variations in the reaction rates over different portions of the confinement vessel which can lead to incomplete reactions and a non-uniform temperature profile. This is particularly true when operating a dense fluidized bed.
FCC units typically include a regenerator, many of which maintain a dense fluidized bed of catalyst particles through which a regeneration gas, such as air, passes to combust coke. The coke forms as a by-product of the cracking operation, and its removal regenerates the catalyst. A common regenerator arrangement introduces a regeneration gas, or air, into the bottom of the regenerator through the bottom closure of the regenerator vessel. The air distribution device divides the air and injects it into the catalyst bed at a multiplicity of points in order to obtain good air distribution. As long as there is no need to withdraw catalyst particles from below the point of air introduction, a simple air distribution device such as a perforated plate or dome over an air chamber will provide efficient and reliable air distribution for the regenerator.
However, the configuration of some FCC process flow arrangements require the removal of catalyst through the bottom closure of the regenerator. The need to withdraw catalyst from the bottom closure of the regenerator complicates the design of the air distribution device. The design of a reliable air distribution device is further complicated by regenerator operating temperatures that normally exceed 705.degree. C. (1300.degree. F.). These temperatures greatly reduce the strength of the materials from which the air distribution devices can be fabricated.
A variety of distribution device designs have been used that will permit the introduction of air and the withdrawal of catalyst from the bottom of the regenerator. One design was the modification of a full plate or dome type air distribution device to include a conduit that extended through the air distribution chamber and communicated a catalyst withdrawal point on the bottom closure with a collection point above the top dome or plate. In this arrangement, the conduit pierced the dome or plate. In order to prevent air leakage around and catalyst movement through the opening for the conduit, a seal bridged the opening between the outer conduit wall and the plate or dome. Catalyst induced erosion and the accumulation of fine catalyst particles made this seal prone to failure. Providing the catalyst collection area above the grid also blocked a significant portion of the distributor cross-section thereby interfering with air distribution.
In order to avoid the problems associated with the seal and to allow free passage of solid particles to a withdrawal point located below the point of air distribution, distribution devices consisting of a planar network or grid of horizontal pipe sections with air outlet nozzles spaced along the pipes have been used. Structural difficulties are often encountered with these pipe type grids. Such problems include weld cracking, metal erosion and warping of pipe sections, as well as the complete detachment or loss of pipe components. Although attempts were made to strengthen the pipe type grid, failure of stronger pipe components still occurred. The inability of stronger pipe components to remedy the problems is believed to stem from the fact that stresses which cause pipe warpage and cracking are typically generated by temperature differentials over the pipe components. Thus, strengthening the grid only serves to intensify the stresses and exacerbate the problems.
Cognizant of the fact that at least some of the stresses leading to failure of air grid components are thermally induced, more flexible designs for air distribution devices have been sought. One such design uses a combination of a dome and radially extending pipe branches to distribute air over the entire regeneration cross-section. This design provides flexibility by using, as a dome, a shallow dish head having a diameter smaller than the diameter of the regenerator vessel. The dome is often supported by a frusto-conical reducer section which decreases the diameter of the dome down to a smaller diameter section which is attached to the bottom of the regenerator closure. A relatively thin wall section and gradual taper of the frusto-conical section provide flexibility to allow for differential thermal expansions in the dome and reducer sections which are induced by temperature gradients and varying expansion rates. The reducer section allows an open space to be maintained between the outside diameter of the frusto-conical section and the end closure of regenerator so that fluidized particles can flow around the dome and into a catalyst withdrawal point. An evenly spaced series of orifices or nozzles distributed over the top of the dome distribute air uniformly over the cross-section of the regenerator lying above the dome.
The remaining cross-section of the regenerator, which is not above the dome, receives a uniformly distributed flow of air through the radially extending pipe branches. Orifices or nozzles are spaced along the branch pipes to provide outlets for the air. The pipe branches project from a cylindrical band which extends vertically and is located between the dome and frusto-conical section. Geometric discontinuities such as sharp corners or junctions between connecting components will multiply the magnitude of thermally or pressure induced stresses. In order to avoid such discontinuities between the vertical band, dome or reducer section, a large radius transition section or knuckle is provided at such junctions. Although the dome and branch pipe style air distribution device did alleviate some of the structural problems generally associated with the air distributors, small cracks in the junction between the band and the dome, and the band and the branch arms persisted in some cases. In addition, erosion of the dome and pipe arm material continued to be a problem. One source of the erosion appeared to be the result of a differential pressure between the outlets on the top of the dome and the outlets on the branch arms which aspirated catalyst into the interior of the dome through the branch arm openings and out through the holes on the dome.
A new attachment arrangement has been discovered for connecting the pipe branches in a dome and pipe branch type air distribution device. This new connection alleviates the cracking problems sometimes associated with the band to dome and band to branch pipe junction while also raising the elevation of the pipe arm outlets relative to the dome outlets so that the beforementioned aspiration of solid particles will not occur.